COMMON" THINGS SPECTACLES. 



the right eye is more perfect than that of the humours of the left 

 eye, for the space to the right of always appears less bright 

 than the space to the left of it. 



19. To apply this instrument for the purpose of adapting spec- 

 tacle lenses to eyes of unequal powers of vision, it is necessary 

 first to ascertain the existence of the inequality of power in the 

 manner already explained. It would then be necessary to provide 

 two distinct screens on which similar scales might be drawn, so 

 that they might be placed at different distances from the aperture 

 M N. Let their relative distances be then determined, so that 

 the two eyes would see the scales with equal distinctness. These 

 distances will then represent the focal lengths of the divergent 

 lenses which it would be necessary to provide for the eyes, so as. 

 to make them see different objects with equal distinctness. 



In the case of weak-sighted eyes, this method will not be appli- 

 cable. In that case let the two screens be placed at equal dis- 

 tances from the aperture M N, and let lenses be selected for each 

 eye separately, closing the other, so as to give a distinct perception 

 of the scales. The two lenses being then simultaneously applied 

 to the eyes, let the scale be viewed with both eyes open. If the 

 lenses be adapted to correct the defect of vision, the two parts of 

 the scale to the right and to the left of 0, seen at the same time by 

 each eye alone, will appear of uniform brilliancy and distinctness. 



If defective eyes were tested by this method, I believe it would 

 be found that inequality of vision would be much more common 

 than is generally supposed, and accordingly the adaptation of 

 spectacles would be considerably improved. 



20. Cases occur not only in which the comparative powers of 

 vision of the two eyes differ, but in which the power of vision, 

 even of the same eye, is different when estimated in different 

 directions. 



I have known short-sighted persons who were more short- 

 sighted for objects taken in a vertical than in a horizontal direc- 

 tion. Thus with them the height of an object would be more 

 perceptible than its breadth, and in general, vertical dimensions 

 more clearly seen than horizontal. This difference arises from 

 the refractive power of the eye taken in vertical planes being 

 different from the refractive power taken in horizontal planes ; 

 and the defect is accordingly removed by the use of lenses whose 

 curvatures, measured in their vertical direction, is different from 

 their curvature measured in their horizontal direction. The 

 lenses, in fact, instead of having spherical surfaces, have elliptical 

 surfaces, the excentricities of which correspond with the varia- 

 tion of the refractive power of the eye. 



